The invention relates to a filter element for use in a unit to filter or purify liquid. In particular, the invention is directed to a filter element having a means for reducing gas entrapment in the filter element during precoating and filtration.
Methods are known in the art for purifying liquids by passing them through a filter element which has been precoated with a layer of ion exchange resin particles, such as a precoat medium in the size range of 60 to 400 mesh, as disclosed in U.S. Pat. No. 3,250,703, issued May 10, 1966, and assigned to the assignee of this invention.
In a typical system of this type a plurality of filter elements are mounted within a filter tank. These filter elements of the prior art generally include a stainless steel core having perforations or openings therein, a layer of coarse wire screen positioned about the core, and a layer of fine mesh wire screen surrounding the coarse screen for supporting the precoat filter medium. A precoat layer of filter medium is deposited on the upstream sides of filter elements by passing a water slurry of filter particles through the filter tank. The length of these filter elements is limited by the expense of stainless elements and the size of the tanks that can be effectively used for such filtration. Because of this size limitation, it is desirable to precoat the entire filter element with a filter medium.
However, during filling of the filter tank with liquid in order to deposit a slurry of precoat particles or to commence purification of liquid through the filter elements, a certain amount of air or other gases typically accumulates in the upper portion of each filter and is trapped because of capillarity. As the pores of the filter element are wetted by the liquid in the tank, an interface is defined between the liquid, air or other gases in the filter tank, and the walls of each pore in the wire screen supporting the filter medium. The surface tension of the liquid across the pore creates a force that must be overcome for gas to pass through the pore. The net gas pressure in the pore which is in equilibrium with the surface tension force caused by capillarity at the largest pore in a filter element is defined as the bubble point pressure, according to Aerospace Recommended Practice (ARP) 901 of the Society of Automotive Engineers, Inc. The lower the bubble point pressure for a given filter element, the lower the probability that gas will be trapped in a filter element during precoating or filtration.
It is known in the art that this entrapped gas has at least two deleterious effects on the filtration system. First, the entrapped gas prevents the flow of slurry through and precoating of those areas of the filter elements in which the gas is present, thereby creating unprecoated areas in which liquid with impurities can pass through the filter element. Second, the presence of entrapped gas during the filtering cycle of the system allows the gas to be periodically released outwardly through the filter elements, which may disrupt or even remove portions of the precoated filter material.
The problem of entrapped gas within filter elements is especially serious in filter elements with absolute particle retention abilities less than 50 microns and with precoat filter media having low pressure drop characteristics, such as powdered ion exchange resins in the size range of 60 to 400 mesh, or smaller. The absolute particle retention ability is typically defined by the minimum dimension of the largest pore in the element. Several attempts to overcome the problem of accumulated gas have been disclosed and known in the prior art, such as described in U.S. Pat. No. 3,680,700, issued Aug. 1, 1972 and U.S. Pat. No. 3,779,386, issued Dec. 19, 1973, both of which are assigned to the assignee of this invention.
With the elements of the prior art a significant volume of entrapped gas accumulates at the top of each element. If the elements are mounted in a filter tank having a bottom tube sheet or plate between an upper influent compartment and a lower filtrate compartment, the gas is trapped within the element core. If the filter tank has a top tube sheet, the gas is trapped outside each filter element. The outside surface area of each element adjacent the entrapped gas volume is not precoated. When the pressure in the filter tank increases above the pressure during precoating causing the entrapped gas volume to be compressed or the force of capillarity to be exceeded or both, unpurified liquid flows through the exposed unprecoated portion of the filter element. The filter tank pressure increases typically in three instances: first, when service flow commences; second, if a flow surge occurs during service flow; and third, when undissolved impurities build up significantly on the filter element. The flow of unpurified liquid through the exposed portion of the element can result in plugging of the element by particulates in the water or premature termination of a filtering cycle by the presence of impurities in the effluent from the filter system.
As disclosed in the above-mentioned patent references, attempts have been made in the prior art to overcome the problem of entrapped gas. For instance, U.S. Pat. Nos. 3,680,700 and 3,779,386 disclose a precoat filter element having a dome-shaped cover or vent sleeve around the upper portion of the element. The cover includes a vent hole communicating with the internal portion of the filter elements. While this method is practical with filter elements for general use with filter media in a size range of 60 to 400 mesh, it cannot be effectively used with a filter element having a low absolute particle retention ability without substantial modification of the apparatus. When an absolute particle retention ability of 50 microns or less is required and a wire mesh cloth is used to achieve this rating, such a filter element has been uneconomical because of the expense of material involved in providing the necessary vent sleeve and the diminution in effective filtering area of the filter element because of the length of the vent sleeve. This diminution is illustrated by the critical length for sizing a vent sleeve, defined by the height of the volume of entrapped gas which will be present after the liquid fill step used in conjunction with the filter tank if no vent sleeve were provided. This height of the volume of entrapped gas is measured as the vertical distance from gas-liquid interface to the highest exposed portion of the filter medium support layer at the top of the filter element, and this height is proportional to the bubble point pressure of the layer of the element having the highest bubble point pressure, typically the outermost layer which supports the filter medium.
Other attempts to overcome the problem of entrapped gas in the prior art include addition of a surface-active agent to reduce the surface tension of the liquid, and a complete drying of the filter element before filling with liquid, to eliminate the liquid providing the surface tension force across an element pore. Addition of a surface-active agent is not an acceptable solution to the problem because the surfactant promotes an unacceptable degree of foaming and the addition of such chemicals as surfactants to the liquid to be purified is often undesirable. Complete drying of the filter element before liquid filling does eliminate entrapped gas, but utilizes a significant amount of time, thereby lengthening the down time of a filter tank system and reducing the amount of time during which the system can be used for filtration.